This invention relates generally to the recovery of marketable products such as oil and gas from substantially fluid impermeable oil shale deposits of kerogen in an inorganic matrix by the application of electromagnetic energy to heat the deposits. More specifically, the invention relates to a method for recovering shale oil from such formations by controlled electromagnetic heating to pyrolyze the kerogen to gas and shale oil at pressures sufficient to drive out the oil, while controlling the electromagnetic power to limit the temperature rise to keep down wasteful coking and cracking. The invention relates to such method including use of a high power radio frequency signal generator and an arrangement of elongated electrodes inserted in the earth formations for applying electromagnetic energy to provide controlled heating of the formations.
Vast amounts of hydrocarbons are contained in deposits from which they cannot be produced by conventional oil production techniques because the hydrocarbon deposits are not fluid and/or the formations are substantially fluid impermeable. Such deposits include oil shales.
It is well known to mine oil shale, heating the mined oil shale on the surface of the earth to an appropriate temperature, and recovering the products thereupon released from the matrix by pyrolysis or distillation. The volume of material to be handled, as compared to the amount of recovered product, is relatively large. Material handling of oil shale is particularly difficult even under the best of conditions, and the problems of waste disposal are substantial.
A number of proposals have been made for in situ methods of processing and recovering valuable products from hydrocarbonaceous deposits. Such methods may involve underground heating or retorting of material in place, with little or no mining or disposal of solid material in the formation. Valuable constituents of the formation, including products of pyrolysis, may be drawn to the surface by a pumping system or forced to the surface by injecting another substance into the formation. It is important to the success of such methods that the amount of energy required to effect the extraction be minimized.
It has been known to heat relatively large volumes of hydrocarbonaceous formations in situ using radio frequency energy. This is disclosed in Bridges and Taflove U.S. Pat. No. Re. 30,738. That patent discloses a system and method for in situ heat processing of hydrocarbonaceous earth formations wherein a plurality of conductive means are inserted in the formations and bound a particular volume of the formations. As used therein, the term "bounding a particular volume" was intended to mean that the volume was enclosed on at least two sides thereof. In the most practical implementations, the enclosed sides were enclosed in an electrical sense, and the conductors forming a particular side could be an array of spaced conductors. Electrical excitation means were provided for establishing alternating electric fields in the volume. The frequency of the excitation means was selected as a function of the dimensions of the bounded volume so as to establish a substantially non-radiating electric field which was substantially confined in such volume. In this manner, volumetric dielectric heating of the formations occurred to effect approximately uniform heating of the volume.
In the preferred embodiment of the system described in that patent, the frequency of the excitation was in the radio frequency range and had a frequency between about 100 KHz and 100 MHz. In that embodiment, the conductive means comprised conductors disposed in respective opposing spaced rows of boreholes in the formations. One structure employed three spaced rows of conductors which formed a triplate type of waveguide structure. The stated excitation was applied as a voltage, for example, between different groups of the conductive means or as a dipole source, or as a current which excited at least one current loop in the volume. Particularly as the energy was coupled to the formations from electric fields created between respective conductors, such conductors were, and are, often referred to as electrodes. The reissue patent disclosed application of the triplate heating method to oil shales at columns 15 and 16, mentioning pyrolysis in the range of 400.degree. C. to 500.degree. C.
Dauphine U.S. Pat. No. 4,193,451 suggested a temperature range on the order of 200.degree. C. to 360.degree. C. for the thermal decomposition of kerogen to produce shale oil and gases using RF energy. Dauphine recognized that shale was generally impervious without suitable fractures and suggested fracturing methods to enhance flow of the shale oil and gases toward one of the wells. More specifically, Dauphine suggested fracturing the shale by the application of RF heating and then maintaining pressure to keep the resulting fissures open, as by external fluid injection or the vaporization of water and/or hydrocarbons and/or the decomposition of temperature sensitive carbonate minerals.